The performance of electronic equipment such as computers and communication devices continues to improve. Generally, designers strive to decrease the size and power consumption of such equipment, while increasing the speed and functionality of this equipment. Decreasing power consumption is a major technical issue facing the semiconductor industry. Two areas of power consumption that have been targeted by designers are losses due to device switching and losses associated with quiescent power consumption commonly referred to as “leakage power.” Curtailing leakage power plays an important role in the design process. Leakage power is emerging as a new critical challenge in the design of high performance integrated circuits (IC)s.
More specifically, the portion of total power consumption of very large scale integration (VLSI) circuits that is caused by leakage currents is increasing dramatically as technology generations transition to processes that create smaller devices. This evolution is often referred to as technology scaling. Currently, power dissipation created by device switching encompasses a significant portion of total power consumption for ICs while leakage power accounts for another significant portion of the overall power dissipation within ICs. Leakage power is particularly important during standby operation of battery-powered, handheld devices and densely populated devices that have heat related problems.
Technology scaling relies on the use of low threshold voltage switching devices, such as transistors, to increase chip densities, where over a million transistors can be placed on an integrated circuit. Low threshold voltage switching devices are devices that generally can operate at lower supply voltages, dissipate less power during switching and operate at higher switching speeds than traditional devices. A significant drawback to these low threshold voltage devices is the relatively large leakage currents. In fact, as devices get smaller and smaller leakage currents increase at an exponential rate.
Leakage power is mainly comprised of gate and sub-threshold leakage currents. Gate leakage involves current that tunnels through the gate oxide to the drain and/or source of a transistor and sub-threshold leakage refers to current that flows between the drain and source terminals when a signal at the controlling gate has the transistor turned off. As technology continues to reduce the size of these low threshold voltage devices, leakage currents are expected to increase leakage power toward 40% to 50% of total power consumption, making leakage power a dominant source of power consumption for many integrated circuits.
Most designers utilize computer aided design (CAD) tools to create new ICs that have reduced power consumption. These tools have design “libraries” that provide a defined set of circuits components commonly referred to as cells that are building blocks utilized to create an IC design. During the design process designers may select cells of varying functions, shapes and sizes the library, place the cells into location in a grid format and connect the cells to create the integrated circuit design. Each cell can provide specific circuit functions, such as signal handling functions and when combined with other building blocks such a power system components, the designer can create a functional system. In such a configuration power system components can be distributed throughout an IC. Common design tools include Physical Compiler from Synopsys™ and First Encounter™ by Cadence Inc. To address power consumption issues, distributed power conservation systems have been incorporated into IC designs. Power conservation systems can control power delivery to portions of the IC when these portions are inactive. In particular, contemporary power conservation systems comprise transistors that are integrated into the design of the IC as headers and footers to isolate sections of an IC from power sources when these sections are inactive. Power transistors can be configured either as a header on the supply side of the power delivery system or as a footer on the return side of the power delivery system (i.e. between a ground plane and the cell). In some configurations both a header and footer are utilized to control power to a section of an IC.
However, implementing such a system creates significant challenges and problems. One challenge involves integration of the transistors into the IC design without adversely affecting the operation of the IC and without exceeding the available space on the IC. In addition, designers must determine when to turn sections of the IC on and off such that the speed of operation of the IC is not significantly impacted.
Another challenge is to design a power delivery system that does not result in increased power consumption. A typical power conservation system can require hundreds of power transistors (i.e. headers and footers), numerous control circuits, and hundreds of power lines. The power conservation system can consume a significant amount of power and, due to the area required by the power conservation system, increase the average lengths of conductive lines in the IC. Increasing the average lengths of the lines not only increases power consumption of the IC but can slow down the speed of operations within the IC. Designers must also carefully locate components of such a bulky system to control interference and provide efficient operation of the IC.
In addition to locating the additional devices and lines, there is a delicate balance between the area of the IC utilized for functionality and the area utilized by the power management system. Space on the IC is limited, and the area utilized by the power management system is not available to efficiently implement functionality. Historically, the actual benefits provided by power management systems are small when compared to the penalties incurred by implementing such a system.
Many electronic products or devices have functional systems on a single microchip or IC. This configuration is often referred to as a “system on a chip” (SoC). A SoC may have circuits such as static random access memory (SRAM), registers, branching logic, multiplexers, and decision logic that accept an input and create an output. Other circuits commonly found on microchips include pipeline circuits and a “one hot property” configuration where flip flops are chained in series and the “one hot” bit advances from flip flop to flip flop. In such configurations not all portions of the circuits operate concurrently during a specific time interval.
Additionally, banks of SRAM cells may share a read path, a write path and/or control lines and such a configuration requires specific portions of SRAM to be inactive when other portions are active. Reducing current to the inactive banks of SRAM can significantly reduce power consumption within an IC. This active-inactive dichotomy also occurs in register files multiplexers, and branching logic.